Jenner, 20, has largely hidden her pregnancy since news broke in September that she was expecting, not even confirming the news until after the birth of her daughter, whose name has yet to be announced.
But it looks like the two had been enjoying being pregnant together, based on a photo Kardashian posted soon after her sister's announcement, showing both of them cradling their baby bumps while clad in loose shirts and underwear.
"Congratulations My Sweet Kylie! What a magical ride it's been!" the Good American designer captioned the red-hued photo. "I'll miss you bumping along with me. Never in a million years did I think we would do this together but it's been that much more special because of it. God is so great!! He had His plan all along!"
It was then that she revealed the two's adorable new nicknames for each other.
"I love you little mama, love big mama," she concluded the caption.
Jenner and Kardashian aren't the only new moms in the KarJenner family.
Sister Kim Kardashian, 37, welcomed daughter Chicago "Chi" West, alongside husband Kanye West in January via a surrogate, due to Kim's complications with her first two pregnancies.
The new baby made her first appearance in the 11-minute video Jenner dropped alongside her birth announcement Sunday, in which the Kylie Cosmetics founder details the moments of her pregnancy fans missed out on while she was staying out of the spotlight.
Just one day before she turns one month old, Chicago can finally be seen on camera for the first time in the video. The pregnant reality star holds her niece reverently. "She's so little," she says.
"Did you figure out the name?" Jenner asks her older sister.
"I think we're going to go with Chicago," Kardashian says with finality.
"I love Chicago," Jenner replies.